THE STRUCTURE AND EVOLUTION OF THIN VISCOUS DISKS .1. NONSTEADY ACCRETION AND EXCRETION

The long-term evolution of solar nebula is a non-steady process. Using the alpha-viscosity model, we computed the long-term non-steady evolu tion of circumstellar disks isolated from neighboring stars. The inner boundary layer, where the pressure is important, was taken fully into account, while the temperature distribution of the disk was assumed t o be constant in time. Computations were made for a variety of angular velocities of a central star. The results indicate that there are thr ee critical angular velocities of the central star, which distinguish between the inward and outward directions of the flows of mass and ang ular momentum. We also found that if the angular velocity of the centr al star is sufficiently high, the disk evolves from accretion to excre tion. From our results we can expect that a protostar continues to acc rete most of the disk mass and reaches a very rapidly rotating state w ith a critical spin rate, if no efficient mechanism works to carry awa y angular momentum from the star. On the other hand, a large mass loss is observed for young stars, such as T Tauri stars, and they are rath er slow rotators. We thus examine how stellar winds keep a protostar a t a low spin rate.